1. Field of the Invention
The present invention relates to a light emitting device capable of color-mixing light emitted by a light emitting element and light obtained through wavelength conversion of a part of the original light, thereby emitting light of a different color.
2. Description of the Related Art
A semiconductor light emitting element such as light emitting diode is small in size, has high power efficiency and emits light with clear color. The semiconductor light emitting element also has such advantages as excellent startup performance and high durability to vibration and repetitive operations of turning on and off. Light emitting devices have been developed that are capable of emitting light of various colors by combining the primary light of the semiconductor light emitting element and a fluorescent material capable of emitting secondary light of a different wavelength through excitation by the primary light, based on the principle of color mixing of light. Such light emitting devices are used as various light sources. Particularly in recent years, as such light emitting devices are spotlighted as the next-generation illumination of lower power consumption and longer service life that replaces the fluorescent lamps, it is required to further improve the output power of light emission and the emission efficiency. There is also a demand for light source of higher brightness in projectors such as headlight of automobile and flood lighting.
Such a light emitting device includes a semiconductor light emitting element die-bonded onto a metallic lead frame or a ceramic substrate and a fluorescent material layer formed around the semiconductor light emitting element by various methods such as potting, screen printing or the like. Japanese Patent Publication No. 3,503,139, for example, discloses a light emitting device that emits white light and has high durability and high color rendering performance, constituted by combining a light emitting diode formed from a gallium nitride-based compound semiconductor that is capable of emitting blue light as the primary light and a garnet fluorescent material activated with cerium that is capable of emitting yellow light as the secondary light. FIG. 10 is a sectional view of a light emitting device 100 disclosed in Japanese Patent Publication No. 3,503,139, that comprises a light emitting diode 102 fastened at a distal end of 105 of a pair of lead frames 105, 106, and a fluorescent material layer 101 that is formed from a resin containing fluorescent material particles and coats the light emitting diode. The light emitting diode 102 and the fluorescent material layer 101 are coated by a bullet-shaped transparent resin 104. Japanese Unexamined Patent Publication (Kokai) No. 2002-141559 discloses such a constitution as a powder of silica, alumina or titania is dispersed as scattering particles in the transparent resin that coats the semiconductor light emitting element and the fluorescent material layer, in order to mitigate color unevenness of the light emitting device that has a semiconductor light emitting element and a fluorescent material layer. Japanese Unexamined Patent Publication (Kokai) No. 2009-24117 discloses such a constitution as fine particles measuring 20 nm or less that is formed from an inorganic material having a high refractive index are dispersed in the transparent resin and in the fluorescent material layer, in order to increase the refractive indices of the transparent resin and the fluorescent material layer thereby to improve light extracting efficiency. According to Japanese Unexamined Patent Publication (Kokai) No. 2009-24117, the particle size of the fine particles is set to 20 nm or less in order to suppress the scattering of light. Japanese Unexamined Patent Publication (Kokai) No. 2008-130279 discloses such a constitution as the particle size of fluorescent material particles contained in the fluorescent material layer is set to a small size that corresponds to the Rayleigh scattering regime, thereby to improve the light extracting efficiency.
However, it has been difficult to improve the color unevenness and light extracting efficiency of the light emitting device at the same time, in the light emitting device of the prior art described above. First, in such a light emitting device of the prior art as that of Japanese Patent Publication No. 3,503,139, the semiconductor light emitting element and the fluorescent material layer are coated with the transparent resin, and epoxy, silicone or the like is often used as the transparent resin. These transparent resins show excellent optical properties such as colorless transparency, homogeneity and high isotropy after curing, although the excellent optical properties allow the emission intensity distribution of the semiconductor light emitting element and the fluorescent material layer to be shown faithfully to the outside. Meanwhile, a difference between the emission intensity distribution of the semiconductor light emitting element and the emission intensity distribution of the fluorescent material layer is likely to arise, and it is not easy to make both emission intensity distributions to be identical. Since chromaticity of light emitted by the light emitting device is determined by the ratio of intensity of the secondary light emitted by the fluorescent material layer to the intensity of primary light emitted by the semiconductor light emitting element, presence of difference in emission intensity distribution between the semiconductor light emitting element and the fluorescent material layer causes the chromaticity to change with the position within the light emitting device and with the direction of viewing the light emitting device, thus resulting in color unevenness.
Japanese Unexamined Patent Publication (Kokai) No. 2002-141559 discloses the constitution where scattering particles are dispersed in the transparent resin that coats the semiconductor light emitting element and the fluorescent material layer, in order to improve the color unevenness. When the scattering particles are dispersed in the transparent resin, light of the semiconductor light emitting element and light of the fluorescent material layer are scattered so that the emission intensity distribution of each member is made more uniform and the color unevenness is suppressed. However, although a large quantity of scattering particles dispersed in the transparent resin can improve the color unevenness, it leads to a problem of lower light extracting efficiency. That is, the scattering particles dispersed in the transparent resin scatter the light of the semiconductor light emitting element and the fluorescent material layer in every direction. As a result, the proportion of light returning to the semiconductor light emitting element and the fluorescent material layer also increases. Light returning to the semiconductor light emitting element and the fluorescent material layer are finally extracted to the outside of the light emitting device after being reflected on various interfaces, while not a small proportion of the light is absorbed in this process. As a result, intensity of light extracted to the outside of the light emitting device at the end decreases, thus resulting in lower light extracting efficiency. In Japanese Unexamined Patent Publication (Kokai) No. 2009-24117 and Japanese Unexamined Patent Publication (Kokai) No. 2008-130279, although the means for improving light extracting efficiency is studied, no consideration is given to the color unevenness caused by the difference between the emission intensity distribution of the semiconductor light emitting element and the emission intensity distribution of the fluorescent material layer.